Electrical sensing apparatus



D. L. ELAM ELECTRICAL sENsING APPARATUS Filed March 11, 1958 mw @A A# wm.NN vu .kmmumwwbmwm Jan. 26, 1960 l Q NNN @7M @fig/f United StatesPatent O ELECTRICAL sENslNG APPARATUS David L. Elam, Roselle, Ill.,assiguor to Electro Products Laboratories, Inc., acorporation ofIllinois Application March 111958, Serial'No. 720,642

3 Claims.Y (Cl. Z50- 27) This invention is concer-ned with an electricalsensing system and more particularly with an electrical sensing systemincorporating a novel indicating circuit.

An electrical sensing apparatus which is responsive to the presence ofconductive material withinV a predetermined area adjacent a pickup isdisclosed and, claimed in copending Elam application, Serial No.481,146, filed January ll, 1955, now Patent No. 2,883,538. In theapparatus disclosed in this application, an oscillator is connected withthe pickup, which includes` a coil forming a portion of the tunedcircuit of the oscil'ator, the circuit sustaining oscillationin theabsence of a conductor in the field of the pickup, but being damped witha conductor present. A circuit connectedv with the oscillator indicatesthe presence or absence of the oscillations in the oscillator.

1t is an object of the present invention to provide a system which givesan indication in response to a predetermined periodic shift inconditionV of the sensing device.

Another object is the provision of a system in which the indication issustained for a period of time greater than the period during which theconductive object is in the field of the pickup, which may be a fractionof a millisecond. i

One feature of the invention is the provision of a system includingsensing means having a rst condition and a second condition andresponsive to a predetermined actuation for shifting from one oftheconditions to the other, indicating means responsive to the sensingmeans and having a first condition and al second. condition, and biasmeans maintaining the indicating means in one of the conditions for aperiod differing from the corresponding period of the sensing means, toprovide a desired type of indication. More particularly, where thesensing means is an oscillator having an oscillatory and anonoscillatory condition, it is a feature of the invention that itprovides a system which is responsive to a shift of the condition of thesensing means between oscillation and nonoscillation.

A further feature is that the bias means maintain the indicating meansin the other, of the conditions thereof when the sensing means remainscontinuously in either of its conditions.

Another feature is that circuit means. are provided to derive a signalfrom the sensing means as a result of the shift in condition thereofwhich signal is utilized to provide a bias potential for the indicatingmeans. Yet a further feature is that the bias circuit has an appreciabletime constant.

Further features and advantages will readily be apparent from thefollowing specification and from the drawings, in which:

Figure l is a schematic circuit diagram, partially in block form, of oneembodiment of the invention; and

Figure 2 is a schematic circuit diagram, partially in I block form, ofanother embodiment ofV the` invention.`

The circuit and apparatus disclosed in. the aforementioned Elamapplication are satisfactory for sensing lenergize the relayl in theplate circuit of the tube.

Patented Jan. 26, 19,60

the presence of a conductive object in the sensitive area of the pickupand have been used as a metal detector, limit switch, counter and thelike. The present invention, with an indicating circuit which isresponsive to a periodic shift of the sensing means from one conditionto another, permits the incorporation of the sensing apparatus in moresophisticated automatic control systems.

Turning now to Figure l of, the drawing, an embodiment of the inventionis illustrated. An oscillatoramplifier unit illustrated in block form at10 has connected thereto a pickup 10a, all of which may be of the typedisclosed in the' aforementioned application. The oscillator-pickup areso designed that the oscillator operates when there is no conductivematerial in the field of the pickup 10a, but the oscillations arequenched when conductive material is in the field. The amplifiedoscillations appear at output terminal 10b of oscillatoramplifer unit 10across resistor 10c in the output circuit of the amplifier. The signalsderived from the oscillatoramplifier unit are utilized, as will appearin more detail later, to provide bias control signals for an outputcircuit including an output tube 11, which has a relay 12 connected inthe anodey circuit thereof. Switch contacts 12aassociated with relay 12may be connected as desired in suitable indicator circuits, to providesignals, such as lighting a light, sounding a buzzer or` the like, inaccordance with the condition being sensed. The terms conductive andnonconductive as used herein in connection with the output tube areintended and should be understood to mean sufficiently andinsufficiently conductive to It will be appreciated that a relativelylow amplitude of current through the tube may be insufficient to actuatethe relay.

The circuit of Figure l is designed to maintain the indicating device,here relay l12, lin an unenergized condition so long as the oscillatorperiodically shifts from a condition of oscillation to no oscillation,at a predeter.

mined rate. An example of an application for this system isindetermining when a moving member, as a shaft, drops below apredetermined speed. The shaft may have mounted on it a toothed wheelwith the pickup 10a located near it so that oscillations are damped eachtime a tooth passes the pickup. So long as the shaft turns at greaterthan the predetermined speed, relay 12 is unenergized, but should thespeed drop below this, or the shaft stop completely, the relay isenergized, giving a desired indication through a circuit (not shown)connected with the contacts 12a thereof.

The circuit and its operation will now be described in detail, andspecific values and type designations assigned to the various elements.-It is to be understood that these values and designations are givensolely for the purpose of disclosing an operative circuit, and manymodications will be apparent to those skilled in the art.

The amplified oscillations appearing at the amplifier output terminal10b are coupled through a capacitor 15, 0.001 ,uf (microfarad), to avoltage doubler circuit including two diodes 16 and 17, each a 1N34A. Solong as the oscillator is operating, i.e. when there is no conductivematerialin the field of pickup 10a, the oscillations are rectified bythe voltage doubler circuit and a negative potential is built up acrosscapacitor 18, 0.002I nf, connected between the control grid of a triode19, one-half of a 6SN7, and ground 25. This negative potential on thegrid keeps the triode 19 near or below cutoff, so that the potential onthe plate thereof is relatively high. When oscillations cease, as byentry of a conductive object in the eld of the pickup, triode 19-conducts and plate volta-ge thereof drops, the resulting negative pulsebeing coupled through capacitor 20, 0.1 pf, to a second voltage doublerrectifier arrangement including two diodes 21, a 1N461 and 22, 1N34A.Thevoutput of this doubler is coupled to a load made up of a parallelcombination of capacitor 23 and resistor 24 connected between thecontrol grid of output tube I11, one-half of a 6SN7, and ground 25. Thenegative pulse resulting from conduction of tube 19 charges capacitor 23applying substantial negative potential to the control gridV of output,tube 11. This potential, while not necessarilyV sufcient to cut off allconduction Vby tube 11 is sufficient to keep the anode current belowthat necessary for energizing relay 12. Immediately following thenegative pulse, the charge Aon capacitor 23 begins to drain off throughresistor 24, connected 'in parallel therewith. Unless another negativepulse renews the charge on capacitor 23 periodically, the Vvoltage onthecontrol grid of tube 11 rises to aY point sutticient to permitenergization of relay 12. p

The permissible time between pulses is a function of the RC timeconstant of capacitor 23 and resistor 24 in parallel with the backresistance of diodes 21 and 22 and is in excess of one RC inrmost casesand may be as much as three RC' depending on the amplitude of the pulsesat the plate of tube 19. For example, for a timing between pulses of onesecond, the product of Rand C should be 0.33.A Such a circuit could havea 37.3 pf capacitor and a 100,000 ohm resistor, or another combinationwith the same time constant. Resistor 24 may be made variable tofacilitate adjustment of the response time of the circuit.

Should the movement being sensed stop completely, :relay 12 is energizedregardless of the condition of the sensing circuit, i.e. whether theoscillator is'operative or inoperative. If the oscillator operatescontinuously, tube Y 19 is cut olf but tube 11 conducts, and relay 12 isenergized. Ify the oscillator is inoperative, tube 19 conductscontinuously, but there are no periodic pulses to control tube-11.Accordingly, relay 12 is again energized. This provides a system inwhich the proper -indi' cation is given regardless ofthe nature of thefault in the movement or operation being'sensed. f

Turning now to Figure 2, another embodiment ofthe invention isillustrated. As in'Figure 1,V the system is based on a sensing circuit10 including an oscillator having pickup 10a connected therewith, inaccordance with the teaching of the aforementioned copendingapplication. Theamplied oscillations derived from output terminal 10b ofthe amplifier-oscillator unit are coupled throughl a bias circuit, aswill appear, to a control tube 30 whichhas a'relay 31 connected in theanode circuit thereof. The circuit of Figure 2 is responsive to a shiftfrom an oscillatory to a nonoscilatory condition kof the sensing circuitto bring output tube 30 into conduction energizing relay 31immediately,n and theV bias circuit keeps he tube conducting suicientlyto maintain'the relay energized for a period Vafter oscillation startsagain. An example of an application of this sensing system` is `incounting rapidlyl moving parts. For example, Where parts are droppingdown a chute, theymovelrelatively rapidly. A pickup yunit associatedwith the chute, as a coil having an open center through which the partsdrop, is triggered for the short period of'time, as a fraction of amillisecond, the part is in the field of the coil. Although theoscillator itself ceases operation during this "xaeaasao p r f acrossamplifier load resistor 10c and is coupled through capacitor 33, 0.001pf, to a rectifier diode 34, 1N34, having a load including capacitor 35,100 auf, in parallel with the resistor 36, 56,000 ohms, connected toground `25. Resistor 37, V100,000 ohms, limits the current ow throughdiode 34. Connected across diode load resistor 36 is a diode 38, 1N459,shunted by resistor 40, with a load comprising capacitor 41, connectedto ground and across which the control grid of control tube 30 isconnected.

During oscillation, diode 34 acts as a half-wave recti- Y lier, thecurrent owing through resistotr 36 with a'negative potential appearingat the -upper terminal thereof Y charging capacitor 41 by a circuitthrough resistor 40, shunted by the back resistance of diode 38. Theresulting negative charge on capacitor 41 holds tube 30 cut oit, or, atleast, keeps the current below that required to maintain relayl 31energized. When a conductiveV article passes the pickup, oscillationceases, and the charge on capacitor 41 drains'ol rapidly through diode38 and resistor 36, reducingA the potential on Ythe control grid ofcontrol tube 30 to zero. The control tube then passes sufficient currentto energize relay 31 providing Vthe desired indication, as to a Ycounterconnected with contacts 31a. When the conductivevarticle has passed thepickup and the oscillator resumes oscillation, the negative po' tentialfrom theY rectified `oscillations appearing'across resistor 36 tends tocharge capacitor 41. However,the resistor 40 and capacitor 41-are ofsuchsize that this requires an appreciable period of time. For example,with resistor 40 having a value of ten megohms and capacitor 411 havingavalue of 0.25,;tf, a delay of two to three seconds in the cutoff of tube30 is possible, allowing the the construction and arrangement may bemade without departing fromf'the spirit and scope of the invention asdisclosed in the appended claims.

I claim: Y Y

l. An electrical sensing system, comprising: sensing means lincluding anoscillator circuit having an oscillatoryY and a non-oscillatorycondition; a biasrcircuit including rectiiier means and a capacitorconnected with said sensing means for developing a negative potentialacross saidl capacitor during the oscillatory condition of saidoscillator circuit, said bias circuit having a negligible time constantwith respect to the sensing period; an amplifier connected with saidbias circuit and rendered cut ofby said negative potential during theoscillatory condition of said oscillator andrendered conductive duringthe non-oscillatory condition of said oscillator, the anode voltage ofAsaid amplier dropping upon the shift from oscillatory-tonon-oscillatory condition; and a second bias circuit connected with theanode of said amplifier and including rectifier means which charge acapacitorv with a negative potential upon the drop in anode voltage; aresistance in parallel with period of time, the action is so rapid thata mechanical l n The output of oscillator-amplifier 10 is developed saidlast mentioned `capacitor for dissipating'said negative potential; andacontrol tube connected withv said last mentioned capacitor and renderednon-conductive Yby said negative potential and conductive in theabsencethereof. 2. A11-,electrical sensing system,v comprising: sensing meansincluding an oscillator circuit having an Yoscillatory and anon-oscillatory condition; a bias circuit connected to said oscillatorincluding a rectier witha parallel connected resistance-capacitanceload, there being a negative potential established across said loadduring the Voscillatory'condition of said oscillator; a seriescircuit ofresistance and capacitance connectedacross said load; a control itubeconnectedV across ysaid second capacitor, `and cut voi in the presenceVYof a negative potentialY thereacross; and a diode in. shunt with saidlast mentioned resaid first mentioned capacitance load; and a controltube 10 connected across the capacitance element of saidresistance-capacitance load of the bias means and controlled by thepotential thereacross.

References Cited in the le of this patent UNITED STATES PATENTS2,590,826 Schenck Mar. 25, 1952 2,806,181 Rockafellow Sept. 10, 19572,807,757 Callinan Sept. 24, 1957

